Since a coating composition containing an organic solvent has many problems such as dangger of firing and explosion, being harmful to humans and creating a public hazard such as environmental pollution and the like, recently, its market share is decreasing and being replaced by aqueous and/or powder coating compositions. However, in a conventional aqueous coating composition, it is essential for the purpose of dissolving or dispersing the vehicle resin in water to introduce a number of hydrophilic groups in the molecule of said resin, use a quantity of neutralizing agent capable of forming water-soluble salt therewith, and to select such resin as having comparatively lower molecular weight, and therefore, the coating film performance in terms of durability, water resistance or the like is not so good. Secondly, since it is impossible to formulate an aqueous coating composition with a high non-volatile content, application characteristics are not, in general, satisfactory. Thirdly, dispersibility of coloring matter is not very good, so that the use is rather limited, especially in such technical fields as requiring a higher order of finishing appearance, gloss and sharpness, as in the automobile industry. An emulsion coating composition whose resin is prepared by the so-called emulsion polymerization in an aqueous medium, has indeed an advantage that a higher molecular resin can be utilized, but at the same time, has such problems that the preparation of said resin necessitates an emulsion polymerization technique of higher standard, and the surfactant or emulsifier added for emulsification will cause additional problems of decreased durability and water resistance of the film. On the other hand, a powder coating composition has its own problems. That is, if the glass transition temperature is too low, there occurs a blocking phenomenon, which will result in a coating with inferior performance, and the necessity of adopting a considerably higher baking temperature and using a special applicator will obstruct the general use of this coating composition.
A coating composition in which the powder coating is dispersed in water as a slurry has also been well known. This comprises as a main ingredient a powder coating. However, in this type of composition, there are problems such that
(1) in general, it is difficult to increase the non-volatile content, and therefore, it is difficult to obtain a thicker coating and a satisfactory adhesion of the film to the substrate, and cracks may easily occur in the film,
(2) it is difficult to have an appropriate viscosity and fluidity necessary for the coating by means of, for example, spraying, and thus, there is a problem of working therewith. The coating is also deficient in gloss.
(3) there is a tendency of sedimentation and separation of solid matter while storing the composition.
(4) Since a dispersing agent is generally used, there are problems in performance of the film such as water resistance and the like.
Among them, poor application characteristics are the most troublesome problem, and the slurry composition has not been practically used up to the present, on this account.
Recently, various attempts have been made to overcome the abovesaid drawbacks of the aqueous dispersion type coating composition. The most attractive one is to add to an aqueous slurry of powder coating a water soluble or water dispersible aqueous resin as a modifier. For example, in Japanese Patent Application Nos. 31636/76 and 25224/76, a water dilutable resin in a quantity of 0.5-30 wt% of resin particles is added, together with 0.05-30 wt% coloring matter, to a composition comprising 90 to 30 parts by weight of aqueous medium and 10 to 70 parts by weight of resin particles having a softening point of 5.degree. to 120.degree. C. and a mean diameter of 0.5 to 80 microns, to improve storage stability of the composition. Japanese Patent Application No. 127151/74 discloses a composition comprising 50 to 80% by weight of powdered resin and 20 to 50% by weight of water, compounded with 0.05 to 5 parts by weight of surfactant and 5 to 50 parts by weight of water soluble or dispersible resin per 100 parts by weight of said powdered resin. Also, in Japanese Patent Application Nos. 74606/79 and 170262/79, and Japanese Patent Publication No. 4149/80, mention is made of a water dispersion type coating composition comprising as main components, powder coating and water soluble resin, the content of said water soluble resin being about 10 to 20% of the powdered resin. In either case, water soluble or water dispersible resin is merely added to the water-powdered resin dispersion system as a modifier which will give the role of dispersion stabilizer. They, therefore, all belong to a category of slurry type coating based on powdered resin and hence, can never be free from the drawbacks possessed by themselves, in substance.
The inventors, as the results of extensive studies on water-borne coatings and especially an aqueous coating composition comprising aqueous resin (water soluble resin or water dilutable resin, hereinafter the same) dissolved or dispersed in water, have first found that it is necessary to use a particular type of aqueous resin which will fulfil certain requirements, one requirement is that water tolerance expressed in terms of water dilution (times) of the resin for a solution incapable of reading out the defined type must be more than 4 in a test wherein 5 g of aqueous varnish of the resin having a normal viscosity usually employed in the manufacture of coating composition are correctly weighed in 100 ml beaker, increasing amounts of deionized water are added thereto, and No. 1 type (26 point type) is read through the beaker. The second requirement is that the surface tension of a 1 wt% aqueous solution is less than 51 dyne/cm. If these requirements are satisfied, then even if the weight ratio of aqueous resin to water insoluble resinous particles (on solid basis) be selected in a wide range of 98:2 to 45:55, there is no undesirable increase in viscosity irrespective of the increased solid content, which is attributable to the improvement of working property of the composition and, furthermore the thus obtained mixture is very stable for a long period of time because of its specific rheological properties, and can result in a film with excellent performance. On the basis of these findings, the inventors have filed a patent application (Japanese Patent Application No. 114686/81).
The abovementioned aqueous coating composition was indeed very successful in improving application characteristic, storage stability, durability, and water resistance of water-borne paint, but was merely developed as surface treatment and primer, like many previous proposals. As a top-coat requiring a highly glossy and flat coating, this composition cannot be said to be fully satisfactory in its desired film performance.
Under the circumstances, it has strongly been desired to have an aqueous type, top-coat paint composition capable of forming a coating with a higher gloss and flatness required for a top-coat, while maintaining the desired properties of aqueous resin in respect of pigment dispersibility and gloss and improving application characteristics (popping and sagging) and storage stability thereof.
The inventors, in a series of studies for attaining the improvement in application characteristics of an aqueous paint composition by adding water insoluble resinous particles, thereby increasing the resin content, have continued studies on the correlation between the film properties and such factors as combination of resins used, properties of water insoluble resinous particles, compounding ratio of aqueous resin to water insoluble resinous particles, and solid content in the resinous composition. The inventors have now found that in a combination of a certain type of aqueous resin and water insoluble resinous particles, when the resinous particles are selected from those whose mean diameter is in a specific range, an aqueous type paint composition which is excellent in application characteristics and storage stability and is useful as a top-coat capable of producing a coating with higher gloss and flat surface can be obtained. Particular preference is given to the composition wherein the solid ratio of aqueous resin and water insoluble resinous particles is in a specific range. On the basis of these findings, the present invention has been completed.